This invention relates to a control system for gasifying fine-grain fuels in a reactor at elevated pressures, and more particularly, to a control system to adjust the supply of such fine-grain fuel delivered as a fluidized flow from a vessel with a vehicle gas into the reactor while a gasification agent is also introduced into the reactor.
Reliable operation of a high-temperature gasifier and more particularly, a high-temperature pressure gasifier, is achieved essentially by accurate metering of the supply of fuel for gasifying and the necessary gasification agents, typically oxygen and steam.
The older types of high-temperature gasifiers operate substantially at atmospheric pressure on the gas side and have a throughput capacity of about 10 tons per hour of fuel. The fuel is coal usually in fine-grain form. An output control for such a gasifier is not used nor is an automatic control used to meter the supply of fuel or each gasification agent. The operation of such a gasifier is based on the concept that the devices for feeding the charge of material into the reactor chamber of the gasifier operate within adequate limits of consistency; consequently, there is no need for accurate determination of the charge of material introduced into the reaction chamber per unit of time, nor for the continuous control of the charge of material so as to maintain some constant value. When the actual values to the charges of material are found to differ from performance requirements, then the devices for feeding the charge of materials into the reaction chamber must be adjusted until the required performance or throughput has been reached. Such a type of control is an open-loop control.
Similar considerations apply to metering the additions of gasifying agents with the fuel. An appropriate quantity of gasifying agent is supplied to the charge of material per unit of time before and/or upon entry as a mixture into the reaction chamber. This depends upon the construction of parts and a particular pressure difference between the gasifying agent feed lines and the reaction chamber. Any corrections or adjustments which become necessary must be made externally in order that the required quantity of gasifying agent is added to the charge of material for introduction into the reaction chamber.
A more modern-type of high-temperature gasifier is operated at reaction chamber pressures of 20 bar or more. As the pressure increases in the reaction chamber, gasifier performance for a given reactor cross section, multiplies. Thus, where high gasification reactor performances are required, only high-temperature and high pressure gasifiers are used.
It is impossible to make control adjustments based on an open-loop system as hereinbefore described for the modern-type of high-temperature and high pressure gasifier because of the greatly-increased performance for a given cross section of reactor and the large quantities of fuel and gasification agent which are introduced into the gasifier per unit of time. This is because the response time of such an open-loop control is excessive even though an immediate action may be taken by operating personnel in charge of controlling the gasification plant. If an excessive amount of gasifying agent, i.e., the supply of oxygen, is fed into a gasifier for as little as a few seconds, there is an immediate and excessive temperature overshoot. Conversely, a deficient addition of gasifying agent causes an immediate temperature drop. In both instances, the improper supply of gasifying agent to the gasifier produces an unacceptable variation in the composition of the end-product gas from the gasifier.
A very effective control system for accurate operation and a very short response time is needed to alleviate these disadvantages. An important consideration in pressure gasifying systems is the ability to control the quantity of gas produced per unit of time from a low value up to a maxium value. The quantity of fine-grain fuel to be fed into the gasifier varies in accordance with the output requirements from the gasifier. To gasify a given quantity of fine-grain fuel per unit of time requires a given input volume of gasifying agent.